The biological and environmental importance of the ozone layer in acting as a protective filter of UV solar radiation is known. According to James Andersen [I. N. Levine, Fisico-quimica, McGraw-Hill, Bogota, 1981], if the amount of stratospheric ozone were to be reduced by only 10% the consequences would be extremely serious for plants and animals.
There have been shown to exist chemical species which destroy the ozone catalytically, such as chlorine atoms, hydrogen atoms, hydroxyl radicals, nitrogen monoxide and bromine atoms. In the case of chlorine atoms, and in the presence of sufficient free oxygen atoms, the decomposition takes place according to the following reactions: EQU Cl.sup.- +O.sub.3 .fwdarw.ClO.sup.- O.sub.2 EQU ClO.sup.- +O.fwdarw.Cl.sup.- +O.sub.2
In the presence of few free oxygen atoms, as is the case of the stratosphere over the Antarctic continent, the mechanism of decomposition is as follows: EQU 2(Cl.sup.- +O.sub.3 .fwdarw.ClO.sup.- +O.sub.2) EQU ClO.sup.- +ClO.sup.- .fwdarw.Cl.sub.2 O.sub.2 EQU Cl.sub.2 O.sub.2 +hv.fwdarw.Cl.sup.- +ClOO EQU ClOO.fwdarw.Cl.sup.- +O.sub.2
The chlorofluorocarbons, CFCs or freons, especially those with long lives such as trichlorofluoromethane (CFC 10 11), dichlorodifluoromethane (CFC 12) and 1,1,2-trichloro-1,2,2-trifluorethane (CFC 113), are the ones mainly responsible for destruction of the stratospheric ozone layer [M. J. Molins & F. S. Rowland, Nature, 249 (1974) 810].
When these chlorofluorocarbons reach the stratosphere, and despite their stability under ordinary conditions, they undergo photolysis due to the action of UV radiation, with the consequent release of chlorine atomic radicals, which act as catalysts in breakdown of the ozone into oxygen in accordance with the above reactions.
The atoms of chlorine are not destroyed following the reaction, but recover and accumulate with any subsequent supply, thereby increasing their potential for destruction of the stratospheric ozone.
Moreover, the CFCs also contribute significantly to the greenhouse effect which threatens serious climatic change within only a few decades.
In 1977 the US government prohibited the use of CFCs as aerosol propellants. In 1987 the Montreal Protocol revealed the intention of most world governments to prohibit the production, distribution and emission into the atmosphere of CFCs [The Montreal Protocol. A Briefing Book, Alliance for Responsible CFC Policy, Rosslyn, Va., 1987].
Pursuant to that objective, intense research work has been carried out into obtaining substitutes for these compounds, and this has led, for example, to the compound known as HFC 134a (CF.sub.3 --CH.sub.2 F), which is non-chlorinated.
Furthermore, equipment has been developed for the recovery and recycling of CFCs, with good results [Primera Red de Reciclaje de los CFC, Quimica 2000, 46 (1990) 16]. Preventing the emission of CFCs into the atmosphere nevertheless calls for their destruction, given that their warehousing and recycling only postpones the problem and residual emissions nearly always exist. In any case, no solution has yet been found for the definitive destruction of these hazardous pollutants.
From the chemical point of view, the CFCs are capable of forming hydrogen acids by the action of water on steel receptacles, with consequent corrosion of same [MATHESON, Gas Data Book, 6th edition, 1980, pp. 671-683].
Processes based on the destruction of CFCs, such as high-temperature incineration, molecular breakdown by plasma, thermocatalytic destruction, photochemical reactions and supercritical water, are -too expensive and/or present technical or environmental problems.
Furthermore, many attempts have been made to destroy CFCs by chemical oxidation, but these have not brought acceptable results.
The destruction of CFCs by reduction has not been so widely studied. Attempts at reduction by hydrogen have been made on metallic catalysts supported in oxides or activated carbon [S. D. Witt et al., Heterogeneous Hydrogenolysis of some Fluorocarbons, amongst other papers].
Catalytic hydrogenation methods normally use noble metals, especially palladium, at high temperatures, and in 30 some cases achieve a significant percentage of the completely dechlorinated derivative. A. Oku et al., Complete Destruction of CFCs by Reductive Dehalogenation using Sodium Naphtalenide, Chem. Lett., (1988) 1789 describes the complete dehalogenation of CFC 113 treated with sodium naphtalenide in tetrahydrofuran with tetraglime at 150.degree. C.
Similar treatments were effective with CFC 12 and CFC 22. Reduction of CFC 12 with amalgamated metals in aprotic solvents gave tetrafluoroethylene [Belgian Patent 751481 (1969)]. That work showed that destruction of CFCs by reduction is a technically viable method, although a disadvantage lay in the use of reagents and/or solvents which were expensive for practical application on an industrial scale.
Studies carried out with carbon tetrachloride and freon 113 [A. P. Tomilov et al., Electrochemistry of Organic Compounds] show that injection of an electron from the cathode destabilizes the carbon-halogen bond and breaks it in the end, freeing a radical and the halide. The radicals can be hydrogenated, normally by capturing a new electron and protonizing or polymerizing. Using this process, cathodic dehalogenation of the carbon tetrachloride and of the CFC 113 itself has been achieved, electrocatalysed by the zinc. In the first case chloroform or methylene chloride was obtained, while in the second chlorotrifluoroethylene, an industrial monomer, was synthesized by partial dechlorination of said CFC in ethanol-water mixtures.
Indirect electrochemical reduction of 1,1,2-trichloro-1,2,2-trifluoroethane (CFC 113) [A. Savall, S. Dalbera, R. Abdelhedi and M. L. Bouguerra, Electrochimia Acta, 35, 11/12, 1727-1737 (1990)] was carried out in a water-alcohol medium (50-90% by volume of methanol) with NH.sub.4 Cl 1M and CFC 113 saturate. The cathode used was of solid zinc, in contact with the solution. The final product obtained was the industrial monomer chlorotrifluoroethylene (CTFE).
For solutions of methanol of less than 70% by volume and weak saturation concentrations of CFC 113, the reaction was limited by the transfer of matter. Under these conditions at a constant overvoltage, the reduction current increased when the concentration of NH.sub.4 Cl altered from 0.2 to 1 M.
For methanol-rich solutions (over 70% by volume) and strongly concentrated CFC 113 at saturation, the reduction reaction was limited by the transfer of charge.
For intermediate solutions of CFC 113, a sufficient increase of the NH.sub.4 Cl concentration led to transition from a charge transfer reaction limitation to a matter transfer limitation.
The reduction of CFC 113 can arise directly by electrolysis on the Zn deposit, according to the following reaction: EQU Cl.sub.2 FC--CF.sub.2 Cl+2e.sup.- .fwdarw.ClFC.dbd.CF.sub.2 +2Cl.sup.-
or by the chemical reaction of the Zn: EQU Cl.sub.2 FC--CF.sub.2 Cl+Zn.fwdarw.ClFC.dbd.CF.sub.2 +ZnCl.sub.2
The catalytic action of the NH.sub.4.sup.+ is not a direct consequence of its acid character. The action mechanism of this ion would appear to be: EQU 2NH.sub.4.sup.+ +2e.sup.- .fwdarw.2NH.sub.4.sup.o EQU 2NH.sub.4.sup.o +Cl.sub.2 FC--CF.sub.2 Cl.fwdarw.ClFC.dbd.CF.sub.2 +2NH.sub.4.sup.+ +2Cl.sup.-
This last reaction is particularly accelerated in the case of metals such as cadmium and zinc with high hydrogen overpressure.
One disadvantage of said process, however, is that the Zn electrodes are totally corroded, presenting major changes in their structure. This is due mainly to the chemical oxidation mechanisms, to the action of the CFC 113 and, finally, to the acidity of the NH.sub.4 Cl.
It is also important to point out that although the literature does contain descriptions of obtaining CTFE by electroreduction of the CFC 113 on Hg, Pb, Zn, Ni, Cu, Cd and Al cathodes and on hydrophobized porous cathodes using aqueous solutions of ammonium salts, detergents and metallic halides (whether or not in the presence of organic solvents such as methanol, ethanol, acetone or dioxane), in none of these electrochemical reductions of the CFC 113 did a greater degree of dechlorination take place than that pertaining to the CTFE, so that no compound such as TFE or halogenated compounds with fewer fluorine atoms was described.
The mechanism of the process for reduction of CFC 113 to produce CTFE appears to be quite similar to that of reduction of dichloro-tetrafluoro-ethane (CFC 114) to obtain tetrafluoroethylene on Hg cathodes [Montecatini Edison S.p.a. Ital. 852.487 (1969) and S. Dapperheld, EP 334.796 (1989)]. Tetrafluoroethylene can also be obtained by electrochemical reduction of dichlorodifluoromethane (CFC 112) on amalgamated Cu in solutions of LiClO.sub.4 0.5 mol dm.sup.-3 (or tetramethylammonium trifluoracetate) by using organic solvents (ethanol, acetonitryl or tetrahydrofuran) at a temperature of -70.degree. C. [N. S. Stepanova, M. M. Gol'din and L. G. Feoktistov, Soviet. Electrochem., 12 (1976) 1070]. In this last case, tetraf luoroethylene is obtained together with chlorodifluoromethane, both in similar proportions. Also described is the obtaining of dichlorofluoromethane, chlorofluoromethane and fluoromethane by electroreduction of trichlorofluoromethane (CFC 11) and the obtaining of chlorof luoromethane and difluoromethane by electroreduction of CFC 12 on Hg cathodes in ethanol-water solutions [Montecatini Edison S.p.a. Ital. 852.487 (1969)].
Throughout the entire literature on electrochemical reduction of CFCs use is made of inert electrodes such as Pt, on which Cl.sub.2 (from Cl.sup.-) is given off, or else anodes which permit fixing of the Cl.sup.- formed by reduction of the CFC (such as Pb anodes).
Russian researchers have tackled the problem of the scant solubility of CFC 113 in water by reducing it on gas-diffusion cathodes made of carbon or copper and hydrophobic fluoropolymer [V. L. Kornienko, G. A. Kolyagin, G. V. Kornienko, Yu. V. Saltykov, Electrokhimiyia, 28, 507-16 (1992), amongst others]. In any case, the efficiency of partial dechlorination of the molecule does not exceed 30-50%.
Other papers and patents have been published on the electroreduction of CFC 113 [S. Dapperheld, EP 334.796 (1989); K. Yagii, H. Oshio, Ger. Offen 2.818.066 (1978); amongst others], but the processes implemented did not achieve complete dechlorination of the initial compounds.